Volume 497 Issue 7450, 23 May 2013

Chiral molecules exist as enantiomers that form non-superimposable mirror images, and chirality has a fundamental role in many aspects of chemistry and biology. It is notoriously difficult to detect and quantify chirality because conventional spectroscopic methods exploit weak effects. Patterson et al. now show that microwave spectroscopy combined with a switched electric field can map the sign of an electric dipole Rabi frequency � a variable that depends directly on the chirality of the molecule � onto the phase of emitted microwave radiation. The effect is then used to determine the chirality of cold gas-phase molecules, illustrated with S- and R enantiomers of 1,2-propanediol and their racemic mixture. The method produces large and definitive signatures of chirality, and is both sensitive and species-selective � making it a potentially ideal and unique tool for determining the chirality of multiple species in a mixture. The red and blue traces depicted in the image come from digitized signals from S and R 1,2-propanediol. Cover design: pikovsky.org

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